The present invention relates to a method for lapping a wafer material and an apparatus therefor. More particularly, the invention relates to an improvement in the method for lapping a wafer material, such as semiconductor silicon wafers and glass plates, with outstandingly high precision in the flatness or planarity of the lapped surface and parallelism of lapped surfaces as well as an apparatus therefor.
Needless to say, various wafer materials, such as single crystal semiconductor silicon wafers, are required to have high planarity of the surface and parallelism between surfaces so that the wafers prepared by slicing a single crystal of silicon as grown must be lapped on the surfaces to be imparted with desired planarity and parallelism. The lapping process of wafers is conventionally performed by using a lapping machine, for example, schematically illustrated in FIG. 1. As is shown in the figure, a wafer W is bonded to a pressing plate 1 by using a mounting material or a wax and the pressing plate 1 bearing a wafer W bonded thereto is placed face-down on a horizontally rotating turn table 3 covered with a buffing cloth 2 so as to have the wafer W contacted with the buffing cloth 2 on the turn table 3. A downwardly opening cup-like top ring 8 is mounted on the pressing plate 1 in such a manner that the periphery of the top ring 8 is in contact with the marginal zone of the pressing plate 1. A weight or weights 4 are mounted on the top ring 8 so that the load of the weights 4 is transmitted to the wafer material W via the top ring 8 and the pressing plate 1. As the turn table 3 is rotated, an aqueous dispersion of a fine abrasive powder 7 is supplied on the turn table 3 from the nozzle 6 to be spread over the buffing cloth 2 so that the lower surface of the wafer W is lapped by the sliding movement relative to the turn table 3 covered with the buffing cloth 2 wet with the abrasive dispersion 7.
FIG. 2 schematically illustrates another lapping machine in which a rotatable top ring 8 provided with inlet and outlet channels of cooling water is mounted on the pressing plate 1, which bears a plural number of wafers W bonded to the lower surface thereof with a rubber-made seal ring 9 interposed therebetween and cooling water is passed through the water-tightly sealed space surrounded by the lower surface of the top ring 8 and the upper surface of the pressing plate 1.
One of the problems in the above described conventional lapping machines is that, since the pressing plate 1 is downwardly pressed by the top ring 8 as is illustrated in FIGS. 1 and 2 only at the marginal zone on the upper surface thereof, the pressing plate 1 is more or less warped to be high at the center as is illustrated in FIGS. 3 and 4 so that the amount of the wafer material removed off from surface of the wafer W by lapping is unavoidably larger in the portions near to the outer periphery of the pressing plate 1 than in the portions toward the center of the pressing plate 1 to cause a decrease in the parallelism between the lapped surfaces.
This problem is particularly serious in recent years with the progress of semiconductor technology toward a higher and higher density of integration in semiconductor-based electronic devices in which a very high degree of parallelism between surfaces of a wafer material as well as flatness of the surface are essential. This problems is accordingly a bottleneck to bar the progress of the semiconductor technology so that is is eagerly desired to develop a method and a lapping machine in which the pressing plate is freed from the problem of warping to ensure high accuracy and precision in lapping of wafer materials.
In view of the above described problem, a proposal has been made in Japanese Patent Kokai 57-194874 according to which the pressing plate 1 in FIG. 1 is unevenly loaded in such a manner that the load on the center of the pressing plate 1 is larger than on the marginal zone thereof. The principle of this improvement is that the pressing plate, which is pivotally supported through a connecting body in a freely swayable and rotatable manner, is provided with a steel ball and a socket in the form of a semispherical cavity to receive the steel ball and the load is transmitted to the connecting body from the pivotal axis through the steel ball. This method or apparatus, however, does not provide a complete solution of the problem due to the concentrated loading on the center portion of the pressing plate.
It would also be a possible way in the lapping machine illustrated in FIG. 2 that the upper surface of the pressing plate 1 is deeply cooled by passing chilled water at a low temperature so as to compensate the upward warping by the thermal contraction of the upper surface of the pressing plate 1 and to keep the flatness of the plate 1. This method, however, is not practicable because a satisfactory flatness of the pressing plate 1 can be ensured only by very carefully controlling a number of parameters including the weight of the top ring 8, number and size of the wafers W, temperature and flow rate of cooling water and so on.
A method is known in the prior art (see Japanese Utility Model Kokai 62-165849), in a process for lapping a wafer material bonded to the lower surface of a rotatable pressing plate by mounting the pressing plate on a horizontally rotating turn table so as to bring the surface of the wafer material into contact with the surface of the turn table and downwardly pressing the pressing plate against the turn table to effect lapping of the surface of the wafer material by the relative sliding movement with the surface of the turn table, the pressing plate is pressed down by interposing a flat bag made of an elastic material and filled with air interposed between the top ring and the pressing plate. The effect of such a bag-interposing method is, however, not quite reliable.